The need and desire to understand and adopt formative assessment practices remain high on the agenda at all levels of educational systems around the world. To advance teachers' use of formative assessment, research attention also needs to be paid to (a) understanding the challenges teachers face when asked to utilize formative assessment practices in subject-specific content areas and (b) to the development of appropriate and sufficiently powerful professional learning designs that can enable change for teachers. To begin addressing these needs, this paper offers a close examination of an intentionally designed professional learning (PL) series to help middle and high school Algebra I teachers understand the formative assessment process and then track and advance their classroom practice. The professional learning design, in this case, is based on a collaborative and formative approach to classroom practice and teacher change with high school mathematics teachers. Together, the PL model and tools provide a formative framework that bridges the theory-practice divide enabling teachers to conceptualize and then plan for, reflect on, and revise the ways in which new formative assessment practices are implemented in their classrooms. Through an analysis of the affordances and constraints of the PL design in practice, this paper provides insights into how discipline-specific professional learning can be better developed and supported throughout the teacher growth process.